Apollo 10

Communications are picked up via the Guaymas tracking station on the west coast of Mexico.

The tripod shaped CM docking probe has now been extended from its retracted launch configuration, in readiness for the transposition and docking with the LM, which will occur shortly after the TLI burn. If this extension had failed, then no docking with the LM 'Snoopy' could have occurred and an alternative mission plan would have had to be adopted.

The purpose of P27 (Program 27) is to insert information into the AGC (Apollo Guidance Computer) memory via a digital uplink transmitted from the ground or via the DSKY keyboard by crew manual input. If manual input is necessary the details would be read up to the crew, who would record this information as a P27 PAD.

Updating the computer's memory is performed by two methods. The first, and the most common is to enter P00 (program 00, which puts the computer into an idle state) and place the Uplink Telemetry switch to Accept. At the start of the update, the computer switches to Program 27 to perform the update. The other, and less common method is to enter Program 27 directly via the DSKY, then enter the data manually using Verb 71. Afterwards, the data is recalled onto the DSKY, one item at a time, to verify it.

On this occasion it will be uplinked from the ground whilst the spacecraft is in communications over the Bermuda ground station.

Gene has copied all the information onto a form that exactly matches the one which Duke was reading from. This version is from the Apollo 11 Flight Plan.

Interpretation of the TLI plus 90 PAD is as follows:

• Purpose: In the event of a serious problem soon after a good Translunar Injection, this PAD gives the details of a burn which would return the spacecraft quickly to Earth. It has an ignition time of 90 minutes after TLI. In the event of such an emergency, the SPS (Service Propulsion System) engine would be used to slow the spacecraft to below Earth escape velocity, resulting in a high altitude ballistic arc that would take them directly to splashdown without any further orbits.
• System: The burn would be under the control of the Guidance and Navigation System (G&N). Burns can also be controlled by the SCS (Stabilization Control System).
• CSM weight (Noun 47): Calculated to be 63,556 pounds.
• Pitch and yaw trim (Noun 48): -1.48° and +1.35°. The SPS engine bell is gimbal mounted to allow its thrust vector to be aligned with the spacecraft's computed centre of mass. Two thumb wheels on the left of the Main Display Console allow adjustment of these pitch and yaw trim angles.
• Time of ignition, T_IG (Noun 33): 3 hours, 55 minutes and 44.87 seconds, Ground Elapsed Time.
• Change in velocity (Noun 81), fps: X, -548.4; Y, +0.1; Z, +6,623.2. This is the change in velocity as defined along three orthogonal axes with respect to the local vertical frame of reference.

The three velocity components in a PAD are always expressed with respect to the local vertical frame of reference. Imagine a coordinate system based on a line from the spacecraft to the centre of the Earth or Moon. (Which you use depends on the sphere of influence you are in, i.e. which gravitational field is stronger.) This line is the Z-axis and is vertical at the point where it intersects the surface. The X-axis is perpendicular to this in whatever direction the spacecraft's orbit is taking it. It is therefore parallel to the local horizontal. The Y-axis is perpendicular to the orbital plane.

In the above burn, we can see that the largest component of velocity change is in the plus-Z direction which is towards the Earth, countering the spacecraft's velocity away from the planet.

• Spacecraft attitude at T_IG: Roll, 180°; Pitch, 239°; Yaw, 1°. Although the velocity change is expressed with respect to the local vertical, spacecraft attitude is expressed with respect to the current alignment of the guidance platform. That alignment is currently set to the launch pad REFSMMAT.
• H_A and H_P (Noun 44): These are the heights of the resulting orbit's apogee and perigee, respectively. Since the computer cannot display an altitude higher than 9999.9 nautical miles, and the maximum height of the CSM during a TLI+90 abort will be much higher than that, H_A is not applicable. The perigee of the "orbit" is targeted at 17.5 nautical miles. A perigee this low will ensure the spacecraft encounters the Earth's atmosphere and cause the spacecraft to re-enter.
• Delta-V_T: 6,645.8 fps. This is the total velocity change that would be experienced by the spacecraft. It is a vector sum of the three components given earlier.
• Burn duration or burn time: 7 minutes, 37 seconds.
• Delta-V_C: 6,619.9 fps. This figure would be entered into the EMS (Entry Monitor System) Delta-V display.

SPS engine burns are normally controlled by the G&N system. If it is a long burn, that is, greater then six seconds, the control is 'closed loop'. The system monitors the achieved Delta-V and shuts down the engine at the appropriate time. In doing this, it takes account of the engine tail-off impulse. It knows how much thrust is expected from this tail-off and can calculate the resulting tail-off Delta-V based on this and the spacecraft mass.

If the G&N system were to fail during a burn, the EMS would provide a backup means of shutting down the engine at the right time. This equipment carries a separate accelerometer which measures Delta-V along the longitudinal axis of the spacecraft. Prior to the burn, the crew enter the expected Delta-V into a display on the EMS. As the burn progresses, the figure showing the remaining Delta-V drops towards zero, at which time the EMS itself shuts down the SPS if the G&N system has not already done so. However, the EMS has no knowledge of the tail-off thrust. The flight controllers take this into account and give the crew a low Delta-V figure for entering into the EMS so that if it is called upon to shut down the engine, it will do so early enough for the tail-off thrust to affect the correct total Delta-V.

• Sextant star: Star number 24, Gienah, should be visible through the sextant when its shaft and trunnion are set to the values 208.7° and 16.7° respectively.

The next five parameters all relate to re-entry, during which an important milestone is "Entry Interface," defined as being 400,000 feet (121.92 km) altitude. In this context, a more important milestone is when atmospheric drag on the spacecraft imparts a deceleration of 0.05 g.

• Expected splashdown point (Noun 61): 26.05° south, 25.00° west; in the mid-Atlantic.
• Range to go at the 0.05 g event: 1,122.9 nautical miles. To set up their EMS (Entry Monitor System) before re-entry, the crew need to know the expected distance the CM would travel from the 0.05 g event to landing. This figure will be decremented by the EMS based on signals from its own accelerometer.
• Expected velocity at the 0.05 g event: 33,993 fps. This is another entry for the EMS. It is entered into the unit's Delta-V counter and will be decremented based on signals from its own accelerometer.
• Predicted GET of 0.05 g event: Expected at 12 hours, 55 minutes and 35 seconds GET.
• GDC Align stars: Stars 43, Deneb (in Cygnus) and 36, Vega (in Lyra) are to be used to align the gyro assemblies if it is not possible to use the guidance platform for this purpose.
• GDC Align angles: X, 067°; Y, 283°; Z, 337°.

The spacecraft has two independent systems for determining attitude and change in attitude. The primary system is the IMU and its freely stable platform. A secondary system, usually tied to the SCS, comprises a set of gyros (BMAG, Body-mounted attitude gyro) attached to the spacecraft structure. Unlike the IMU, which measures absolute attitude, these gyro assemblies measure the rate of attitude change. If need be, absolute attitude can be derived from these but this measurement is imprecise so at regular times, the crew presses the GDC Align button to make the GDCs (Gyro Display Couplers) knowledge of attitude match that from the IMU. In case the IMU is not working, the crew have a backup method of aligning the GDCs by sighting two stars through the scanning telescope in a particular way. They know what the spacecraft's attitude should be when this is achieved and can dial this into the GDCs, properly aligning them.

The final note in the PAD, is No Ullage. Since the SPS propellant tanks are full, there is no need to perform a small RCS burn, known as the ullage burn, to settle their contents.

Additionally, Charlie Duke has given details of a P37 PAD for TLI plus 4 hours. The purpose of P37 (program 37) is to compute a return-to-Earth trajectory with the condition that the CSM is outside the lunar sphere of influence at the time of ignition. The program takes the four values from the PAD; the specified time for ignition of the engine, a specified maximum change in velocity (or Delta-V), the longitude of the splashdown and the GET for the start of re-entry. These act as a set of constraints with which it calculates the desired trajectory and the details of the burn to achieve it.

Details of the P37 PAD are:

• GET_I Ground Elapsed Time of ignition: 6 hours, 30 minutes.
• Delta-V_T: 6,373 fps. This is the specified maximum total velocity change that would be experienced by the spacecraft.
• Longitude: 165° West. This is the longitude of the Earth landing point for entry guidance.
• GET 400k: 22 hours, 21 minutes. Ground Elapsed Time of Entry Interface.]

Further P37 PADs will be read to the crew later to cover much of the outgoing coast.

The TLI Pad details are as follows:

• TB-6 predict light: This comes on at 002:24:23, and T_IG (time of ignition (TLI), 578.6 sec after commencement of TB-6) will therefore be at 002:34:01.6.
• Attitude for TLI: 179°, 116°, 000° in roll, pitch and yaw respectively. This attitude is stated with respect to the orientation that the guidance platform has held since launch.
• Duration of burn: 5 minutes, 43 seconds.
• Delta-V_C: 10,437.6 fps will be entered into the EMS to allow the crew to monitor the remaining velocity to be added.
• V_I: Indicated velocity at engine cut-off is 35,603 fps.
• Separation attitude: The correct attitude for separation of the CSM from the launch vehicle is 358°, 151°, 40° in roll, pitch and yaw respectively with respect to the local vertical/local horizontal. Among the criteria for adopting this attitude is solar illumination of the LM to assist the docking procedure and ensure the crew are not looking directly at the Sun.

An additional comment is that the TLI plus 10-minute abort would require a pitch angle of 267°.

Communications now go through the USNS Vanguard tracking ship.

The CMC (Command Module Computer) is being prepared to receive the P27 update. The CMC must be in an idle status (running P00, essentially a "do-nothing" program) with switches set to accept ground updates. P00 is commonly pronounced "pooh" by all the Apollo crews.

Now that the P27 upload to the CMC has been completed, the switch which was set to Accept is now returned to its default setting of Block, which will stop any unplanned uplink getting in to the computer's memory.

Stafford is requesting the latest update on the orbital parameters that are constantly measured via radar and doppler tracking as the space vehicle passes over the ground stations, especially during the 'stateside' pass.

The pyrotechnic devices which will separate the CSM from the launch vehicle have been armed in case of a serious problem with the S-IVB during the preparation and execution of TLI.

The function of the UDL (Up Data Link) equipment is to receive, verify, and distribute digital updating information sent to the SC by the MSFN at various times throughout the mission to update. The Up Telemetry Command switch being moved to Reset performs a real-time command (RTC) reset function and keeps power applied to the power supply. This resets all RTC relays except those relays affecting the system A abort light and the crew alarm.

During the Technical Debrief in June 1969, the crew commented on the doffing of the helmets and gloves prior to TLI.

Young, from the 1969 Technical debrief: "We had agreed that we wouldn't wear helmet and gloves for TLI and then chickened out, there, at the last moment and put them on. It was more psychological than physiological, because you know if anything had happened, there wouldn't have been anything you could do."

Stafford, from the 1969 Technical debrief: "It was so easy, we were ahead of the timeline, and we had nothing else to do. We said, 'Shall we put them on?'"

Cernan, from the 1969 Technical debrief: " But it was a case of being ahead of it, sitting there and saying, 'Well, why not?'"

Pyro devices are armed just prior to TLI to enable a rapid separation of the CSM from the S-IVB, should the booster stage malfunction.

The ARIA are the Apollo Range Instrumented Aircraft (adapted Boeing KC-135), that were positioned under the space vehicle's groundtrack where there was no ground station coverage. They relayed telemetry, data and voice communications to and from the space vehicle, via the Guam ground station into the MSFN.

Communications are now picked up through an ARIA aircraft.

An unsuccessful attempt is made to establish voice communications through another ARIA aircraft.

The next ground station for communications is Carnarvon in Western Australia.

Time Base 6 provides the relative timing sequence for the restart of the J-2 engine on the S-IVB. 42 seconds into TB-6, the sequence begins by igniting a burner which will heat GH₂ in the Start Tank, to spin up the propellant feed line turbines.

To help them coordinate their activities leading up to TLI and after, the crew preset a timer to 51:00. When they start it, it will count up for nine minutes to 00:00, it should coincide with the starting of the S-IVB engine. The crew start the timer 30 seconds after TB-6 commences, as indicated by a light going out.

Program 47 (P47) is used to monitor the vehicle acceleration during a non GNCS-controlled thrusting manoeuvre and to display the Delta V applied to the vehicle by this thrusting manoeuvre. TLI is primarily controlled by the IU.

The "S-II Sep" light comes on again with 1 minute, 24 seconds to go to ignition. Soon after, the two APS (Auxiliary Propulsion System) modules at the base of the S-IVB fire to settle the contents of the tanks. This ullage burn also provides an initial head of pressure of the propellants towards the engine's turbopumps.

The "S-II Sep" light goes out 18 seconds prior to ignition, to signal the final phase of starting the J-2. On starting this sequence, the supercold fuel (H₂) will flow through the combustion chamber walls for 8 seconds to condition them prior to discharge of the Start Tank's GH₂ through the turbines to spin them up.